Deep water directional wave spectra, measured by an NDBC 3-meter buoy off Montery bay, are transformed to shallow water using a linear refraction model. The transformed directional spectra are compared with measured spectra using pressure gauge arrays in shallow waters at Marina and Santa Cruz. The classical Longuet-Higgins et al. (1963) method of computing directional wave spectra and a new exact Founrier coefficients representation method (Grauzinis, 1989) are used to compute directional wave spectra. The new method of computing directional wave spectra, which represents bimodal distributions of wave energy exactly matching the measured Fourier coefficients to second order, demonstrates improved directional resolution over the classical technique. This work examines the accuracy and limitations of modeling linear refraction by comparing with field observations over complex bathymetry. In general, linear refraction can give reasonable energy and direction estimates starting with deep water spectra, but notable exceptions can occur. The largest prediction error occurred at Marina on 18 January 1988 for the case f a severe storm. This is presumably due to diffractive and non-linear effects of the high waves causing loss of accuracy. The linear refraction model is not suitable for handling such problems.
Thornton, Edward B.
Naval Postgraduate School (U.S.)
Naval Postgraduate School
M.S. in Meteorology
Department of Meteorology
Approved for public release; distribution is unlimited.